1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for recording and/or reproducing information on and/or from an optical disk having a plurality of tracks by using a light beam spot, and more particularly to a velocity detector in such an apparatus for detecting the moving velocity of the light beam spot during a seek operation to direct the light beam spot to a desired track on the optical disk.
2. Related Background Art
In recent advancement of laser technology, the development of a high capacity, high density random access optical disk has been attracting attention. In such an optical disk, a finely focused light beam spot is directed to a desired track on a recording medium (disk) and moved along the desired track to record and/or reproduce information.
Various accessing methods have been deviced. Among others, a cross-track method in which a target track is sought by counting the number of tracks which the light beam spot crosses when it accesses to the target track has been attracting attention because of its high seeking precision.
In such a cross-tracking system, the target track is sought by counting the number of tracks crossed and a signal which relates to the difference between a command velocity and the real velocity of the light beam spot on the disk is supplied to light beam spot moving means such as a linear motor based on the number of tracks crossed in order to control the velocity of the light beam spot.
The command velocity is generated in accordance with a velocity profile generated by a CPU in accordance with a seek distance prior to seeking, and the real velocity may be measured by one of various methods.
An example of real velocity measurement method is disclosed in Japanese Laid-Open Patent Application No. 63-173230. This method is described below.
In this method, a velocity detection method called a period measurement type is used when the moving velocity of the light beam spot is high, and a velocity detection method called a differentiation type is used for a low velocity.
The period measurement type velocity detection method is first explained. FIG. 1 shows a block diagram of the period measurement type velocity detector.
In FIG. 1, when the light beam spot moves across a track, a tracking error signal is in a form of a sine wave. The sine wave signal is digitized and one-period interval of the pulse signal is measured based on a reference clock. A reciprocal of the measured value is determined from a ROM table to produce a velocity signal.
The differentiation type velocity detection method is now explained. FIGS. 2 and 3 show a block diagram of the differentiation type velocity detector and signal waveforms at various points of the block diagram.
In FIGS. 2 and 3, a sine wave tracking error signal 21 generated when the light beam spot crosses the track is converted to differentiated waveforms 22 and 23 by a differentiator and an inverter. On the other hand, a linear range clipper produces timing signals 26 and 27 based on threshold levels 24 and 25 corresponding to a linear range of the tracking error signal and supplies them to switches A and B. The switches A and B close when the timing signal is of high level, and open when the timing signal is of low level. Thus, the switch A transmits only positive peak portions of the differentiated waveform 22 to a succeeding stage by the timing signal 26, and the switch B transmits only positive peak portions of the differentiated waveform 23 to the succeeding stage by the timing signal 27. As a result, a signal 28 which is close to a D.C. signal is produced at an output of a hold amplifier if the frequency of the tracking error signal is constant. Since the output 28 is produced by the differentiation, it is of a high level when the frequency of the tracking error signal is high, and of a low level when the frequency of the tracking error signal is low. That is, the output 28 represents the moving velocity of the light beam spot on the disk.
The reason for using different detection methods for the high moving velocity and the low moving velocity is explained below. The period measurement type velocity detection method generates a significant phase lag in low velocity detection and it is therefore, not suitable to the low velocity detection. However, because of its wide dynamic range, it is effective in other velocity ranges including the high speed range. The differentiation type velocity detection method generates a small phase lag but it is difficult to attain the velocity detection of a wide dynamic range.
By using both methods in a manner to complement the defects of respective methods, velocity detection over the entire velocity can be attained.
However, the above velocity detection methods involves the following problems.
In the velocity detection in the low velocity range, only positive peak areas of the differentiated signal and the inverted signal thereof which correspond to the linear range of the tracking error signal are sampled, and the sampled value is held during the sample-to-sample interval to produce a continuous velocity signal. However, if the velocity changes due to any external disturbance during the sample hold period, correct velocity detection for that velocity change cannot be attained. Further, in the vicinity of the target track, the velocity is low and the period of the differentiated waveform of the tracking error signal is long. As a result, if the target track is reached during the peak hold period or an external disturbance such as vibration is applied, the track pull-in may be done without detecting a correct velocity. This results in inaccurate velocity control of the light beam spot and impedes accurate access to the target track.